The present invention relates to an epoxy resin-based composition suitable for encapsulation of semiconductor devices or, more particularly, to an epoxy resin-based composition for encapsulation of semiconductor devices capable of exhibiting very high flowability in the encapsulating works of semiconductor devices and also capable of giving a molded body with little fins having an outstandingly small coefficient of thermal expansion and high resistance against crack formation.
Various kinds of synthetic resin compositions are known and used in the prior art as a material for encapsulation of semiconductor devices including epoxy resins, silicone resins, diallyl phthalate resins, phenolic resins, polyphenylnene sulfide resins and the like as the principal ingredient thereof. Among them, in particular, phenol-curable epoxy resins are currently used most widely and in the largest quantity in view of the advantages thereof in respect of the moisture resistance, mechanical properties, productivity, cost and so on.
As is known, the encapsulating works of semiconductor devices with a resin composition are mostly undertaken by the techniques of low-pressure transfer molding using a metal mold having cavities each for a piece of the semiconductor device. It is a trend in recent years that larger and larger metal molds are used in the encapsulating works in order to be provided with an increased number of cavities. When a large metal mold with many cavities is used for the encapsulating work, a drawback is sometimes caused that a part of the metal mold or cavities is left unfilled with the resin composition so that it is eagerly desired that the resin composition used for the low-pressure transfer molding should be imparted with particularly high flowability without sacrificing other properties.
In this regard, several proposals have been made for the improvement of phenol-curable epoxy resin-based compositions including, first, use of a resin having a low melt viscosity or admixture of a low-molecular diluent, second, decrease of the amount of the curing catalyst and, third, decrease of the amount of the inorganic filler. These proposed methods each have several disadvantages and problems. For example, the improved flowability of the resin composition obtained by the first method is always accompanied by the increase in the fins by molding and, when a diluent is used, the molded bodies suffer the undesirable phenomenon of bleeding on the surface or decreased dimensional stability as a consequence of the increased content of volatile matters. Moreover, a serious difficulty is sometimes encountered in the accelerated corrosion of the aluminum wirings. The second method naturally results in the decreased curability of the resin composition which is retrogressive against the requirement for an increased curing velocity of the composition from the standpoint of productivity improvement. The third method is also disadvantageous due to the increase in the coefficient of thermal expansion and stress in the molded body of the resin composition. Namely, the molded body is less resistant against crack formation and an increased stress is given to the aluminum wirings and passivation films on the surface of the silicon chip encapsulated with the resin composition in addition to the decrease in the moisture resistance.
It is also known that the stress in the molded body of an epoxy resin-based composition can be decreased by compounding the composition with a quartz powder having a spherical particle form as the inorganic filler. According to the disclosure in Japanese Patent Kokai No. 58-138740, an epoxy resin-based composition may contain from 30 to 80% by weight of a fused silica powder of spherical particle form having an average particle diameter in the range from 1 to 60 .mu.m. Such a resin composition, however, is still not quite satisfactory in respect of the flowability in addition to the relatively low resistance of the molded bodies against crack formation.